Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/ac458564cb95
Author: Ryan VanderMeulen <rvandermeulen@mozilla.com>
Desc: Merge pull request #18284 from azasypkin/bug-980461-download-button-style

Bug 980461 - [MMS] Visual adjustments to attachment download indicator.

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https://hg.mozilla.org/integration/gaia-central/rev/dafff03a6b4e
Author: Aleh Zasypkin <azasypkin@mozilla.com>
Desc: Bug 980461 - [MMS] Visual adjustments to attachment download indicator. r=julien
Bumping manifests a=b2g-bump